1. Field of the Invention
The present invention relates to a drive circuit suitably applicable to a display device that uses, for example, an organic Electro Luminescence (EL) element. The present invention also relates to a display device having the drive circuit.
2. Description of the Related Art
In recent years, in the field of display devices displaying images, a display device that uses, as a light emitting element, an optical element of current-driven type whose light emission intensity changes according to the value of a flowing current, e.g. an organic EL element, has been developed, and its commercialization is proceeding. In contrast to a liquid crystal device and the like, the organic EL element is a self-light-emitting element. Therefore, in the display device using the organic EL element (organic EL display device), gradation of coloring is achieved by controlling the value of a current flowing in the organic EL element.
As a drive system in the organic EL display device, like a liquid crystal display, there are a simple (passive) matrix system and an active matrix system. The former is simple in structure, but has, for example, such a problem that it is difficult to realize a large and high-definition display device. Therefore, currently, development of the active matrix system is brisk. In this system, the current flowing in a light emitting element arranged for each pixel is controlled by a drive transistor.
In the above-mentioned drive transistor, there is a case in which a threshold voltage Vth or a mobility μ changes over time, or varies from pixel to pixel due to variations in production process. When the threshold voltage Vth or the mobility μ varies from pixel to pixel, the value of the current flowing in the drive transistor varies from pixel to pixel and therefore, even when the same voltage is applied to a gate of the drive transistor, the light emission intensity of the organic EL element varies and uniformity of a screen is impaired. Thus, there has been developed a display device in which a correction function to address a change in the threshold voltage Vth or the mobility μ is incorporated (see, for example, Japanese Unexamined Patent Application Publication No. 2008-083272).
A correction to address the change in the threshold voltage Vth or the mobility μ is performed by a pixel circuit provided for each pixel. As illustrated in, for example, FIG. 20, this pixel circuit includes: a drive transistor Tr1 controlling a current flowing in an organic EL element 111, a write transistor Tr2 writing a voltage of a signal line DTL into the drive transistor Tr1, and a holding capacitance Cs, and therefore, the pixel circuit has a 2Tr1C circuit configuration. The drive transistor Tr1 and the write transistor Tr2 are each formed by, for example, an n-channel MOS Thin Film Transistor (TFT).
FIG. 19 illustrates an example of the waveform of a voltage applied to the pixel circuit and an example of a change in each of a gate voltage and a source voltage of the drive transistor. In Part (A) of FIG. 19, there is illustrated a state in which a signal voltage Vsig and an offset voltage Vofs are applied to the signal line DTL. In Part (B) of FIG. 19, there is illustrated a state in which a voltage Vdd for turning on the drive transistor and a voltage Vss for turning off the drive transistor are applied to a write line WSL. In Part (C) of FIG. 19, there is illustrated a state in which a high voltage VccH and a low voltage VccL are applied to a power-source line PSL. Further, in Part (D) and (E) of FIG. 19, there is illustrated a state in which a gate voltage Vg and a source voltage Vs of the drive transistor Tr1 change over time in response to the application of the voltages to the power-source line PSL, the signal line DTL and the write line WSL.
From FIG. 19, it is found that a WS pulse P1 is applied to the write line WSL twice within 1 H, a threshold correction is performed by the first WS pulse P1, and a mobility correction and signal writing are performed by the second WS pulse P1. In other words, in FIG. 19, the WS pulse P1 is used for not only the signal writing but also the threshold correction and the mobility correction of the drive transistor Tr1.
In the following, the threshold correction and the mobility correction of the drive transistor Tr1 will be described. By the application of the second WS pulse P1, the signal voltage Vsig is written into a gate of the drive transistor Tr1. As a result, the drive transistor Tr1 is turned on and a current flows in the drive transistor Tr1. At the time, when a reverse bias is applied to the organic EL element 111, electric charge flowing out from the drive transistor Tr1 fills the holding capacitance Cs and an element capacitance (not illustrated) of the organic EL element 111, causing a rise in the source voltage Vs. When the mobility of the drive transistor Tr1 is high, the current flowing in the drive transistor Tr1 is large and thus, the source voltage Vs rises quickly. On the contrary, when the mobility of the drive transistor Tr1 is low, the current flowing in the drive transistor Tr1 is small and thus, the source voltage Vs rises more slowly than when the mobility of the drive transistor Tr1 is high. Therefore, it may be possible to correct the mobility by adjusting a period of time for correcting the mobility.